1. Field of the Invention
The present invention relates to an organopolysiloxane composition that can be vulcanized at room temperature into an elastomer that polymerizes or crosslinks by polycondensation and that does not contain alkyltin-based catalysts which exhibit toxicity problems.
2. Description of Related Art
The invention also relates to the use of novel catalytic polycondensation compositions which have a carbene structure and are stable in solution, as organopolysiloxane polycondensation reaction catalysts.
Elastomer formulations that crosslink via polycondensation generally involve a silicone oil, generally a polydimethylsiloxane, with hydroxyl end groups, optionally prefunctionalized by a silane so as to have alkoxy ends, a crosslinking agent, a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and other optional additives such as bulking fillers, adhesion promoters, colorants, biocidal agents, etc.
These organopolysiloxane compositions that can be vulcanized at room temperature are well known and are classified into 2 different groups: single-component compositions (RTV-1) and two-component compositions (RTV-2).
The term “RTV” is the acronym for “Room Temperature Vulcanizing”.
During crosslinking, water (either provided by atmospheric moisture in the case of RTV-1 compositions, or introduced into one part of the composition in the case of RTV-2 compositions) enables the polycondensation reaction, which results in the formation of the elastomeric network.
Generally, single-component (RTV-1) compositions crosslink when they are exposed to moisture from the air, that is to say that they cannot crosslink in an enclosed medium. For example, the single-component silicone compositions used as sealants or cold-setting adhesives follow a mechanism of hydrolysis of reactive functional groups of the acetoxysilane, ketiminoxysilane, alkoxysilane, etc. type, followed by condensation reactions between the silanol groups formed and other residual reactive functional groups. The hydrolysis is generally carried out by virtue of water vapor which diffuses into the material from the surface exposed to the atmosphere. Generally, the kinetics of the polycondensation reactions is extremely slow; these reactions are therefore catalyzed by a suitable catalyst. As catalysts which are used, use is most often made of catalysts based on tin, titanium, an amine or compositions of these catalysts. Catalysts based on tin (cf. in particular FR-A-2 557 582) and on titanium (cf. in particular FR-A-2 786 497) are catalysts that are very effective.
As regards two-component compositions, they are sold and stored in the form of two components, a first component containing the base polymer materials and the second component containing the catalyst. The two components are mixed at the moment of use and the mixture crosslinks in the form of a relatively hard elastomer. These two-component compositions are well known and are described, in particular, in the book by Walter Noll “Chemistry and Technology of Silicones” 1968, 2nd Edition, on pages 395 to 398. These compositions essentially comprise 4 different ingredients:                a reactive α,ω-dihydroxydiorganopolysiloxane polymer,        a crosslinking agent, generally a silicate or a polysilicate,        a tin catalyst, and        water.        
Usually, the condensation catalyst is based on an organic tin compound. Indeed, many tin-based catalysts have already been proposed as a catalyst for crosslinking these RTV-1 or RTV-2 compositions. Conventional polycondensation catalysts comprise dialkyltin compounds, in particular dialkyltin dicarboxylates such as dibutyltin dilaurate and diacetate, alkyl titanate compounds such as tetrabutyl or tetraisopropyl titanate, and titanium chelates (EP-A-0 885 933, U.S. Pat. No. 5,519,104, U.S. Pat. No. 4,515,932, U.S. Pat. No. 4,563,498, U.S. Pat. No. 4,528,353).
However, the alkyltin-based catalysts, although very effective, usually colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
Another important aspect for an organopolysiloxane polycondensation reaction catalyst is the pot life, that is to say the time during which the composition can be used after mixing without curing. This time must be long enough to allow it to be used, but short enough to obtain a molded article that can be handled at the latest a few minutes or a few hours after it has been manufactured. The catalyst must thus make it possible to obtain a good compromise between the pot life of the catalyzed mixture and the time at the end of which the molded article can be handled (these times depend on the targeted application such as, for example, the molding or manufacture of seals). In addition, the catalyst must confer, on the catalyzed mixture, a spreading time which does not vary as a function of the storage time.
Application FR 03 41232 relates to a process for preparing a polyorganosiloxane by polymerization via ring opening and/or redistribution of polyorganosiloxane, in the presence of a catalyst (or initiator) consisting of at least one nucleophilic carbene.
Application FR 06 03563 relates to a process for polycondensation of polyorganosiloxanes also using catalysts with a carbene structure.
The carbene catalysts described in the abovementioned two applications are unstable and must generally be either used in a crystalline form, or generated in situ from to precursor salts. As it happens, carbenes are highly reactive species that degrade in the open air, which implies storing them and handling them under an inert atmosphere.